1. Field of the Invention
The present invention relates to the polishing of wafers in the process of fabricating semiconductor devices. More particularly, the present invention relates to the conditioning of the polishing pad of a chemical mechanical polishing apparatus.
2. Description of the Related Art
As semiconductor devices become more highly integrated and more miniaturized, and as the number of layers of interconnections of the devices increase, the step difference at the surface of the device increases. Techniques for planarizing the stepped surface include the re-flowing of spin on glass (SOG) or borophosphosilicate glass (BPSG) and chemical mechanical polishing (CMP). Among these techniques, CMP is an essential process for manufacturing chips on a submicron scale using plasma enhanced chemical vapor deposition (PECVD) and reactive ion etch (RIE) processes.
A conventional chemical mechanical polishing (CMP) apparatus is shown in FIG. 1A. The CMP apparatus includes a polishing table 12, a polishing pad 14 mounted to the table 12, a slurry supply device 16 for supplying a slurry onto the polishing pad 14, a conditioning device 20 comprising a conditioning disc 22 for conditioning the polishing pad 14, a polishing head 18 for fixing a wafer W against the polishing pad 14, and a conditioner bath 24 comprising a tank filled with de-ionized water for cleaning the conditioning disc 22. The polishing table 12, the polishing head 18, and the conditioning disc 22 are driven by motors 11, 17, and 21, respectively.
In CMP, a wafer W is polished chemically by supplying the polishing slurry onto the polishing pad 14 so as to react with the material at the surface of the wafer. The wafer is also polished mechanically by pressing the wafer W against the polishing pad 14 with the polishing head 18 while the polishing head 18 and the polishing pad 14 are rotated. After the wafer W is planarized, the wafer W is transferred. Next, the conditioning disc 22 is moved onto the polished pad 14 to condition the polishing pad 14. After the polishing pad is conditioned, the conditioning disc 22 is soaked in de-ionized water 25 and is held in a stand-by position to prevent polishing slurry or a polishing by-product attached to the conditioning disc 22 from being exposed and being hardened and to remove the polishing solution or the polishing by-product.
However, the polishing slurry or polishing by-product comprises particles that are comparatively large. Thus, as shown in FIG. 1B, these particles 29 remain stuck between artificial diamonds 23 of the conditioning disc 22 even after the conditioning disc 22 has progressed to the stand-by position. Consequently, the polishing slurry or polishing by-product 29 can fall onto the polishing pad 14 during the subsequent conditioning process. Also, the force by which the diamonds 29 themselves are adhered to the conditioning disc 22 is reduced by the slurry so much so that the diamonds 23 can fall onto the polishing pad 14 during the conditioning process. The slurry, the polishing by-product, or materials which can fall onto the polishing pad will be referred to hereinafter as polishing impurities. The polishing impurities will scratch the surface of the wafer during the CMP process, whereby the quality of the semiconductor devices will be lowered, and the yield of the semiconductor devices will be correspondingly reduced.
An object of the present invention is to solve the above-described problem of the prior art by providing a CMP apparatus and method in which polishing impurities remaining on the conditioning disc after the conditioning process are removed from the conditioning disc prior to the next conditioning and hence, polishing process.
The chemical mechanical polishing apparatus includes a polishing pad, a conditioning device comprising a conditioning disc for conditioning the polishing pad, and a conditioner bath comprising a tank of liquid, such as de-ionized water, and a conditioning disc cleaner for cleaning the conditioning disc. The cleaner may be a brush having a plurality of bristles for applying pressure to and contacting the surface of the conditioning disc to remove impurities from the conditioning disc. Alternatively, the cleaner may be an ultrasound device.
The conditioning disc cleaner may be fixed in place in the tank of conditioner bath. In the case in which the conditioning disc cleaner is a brush, the conditioning disc is rotated against the bristles of the brush. On the other hand, the conditioning disc cleaner may include a motor for rotating the brush while the bristles of the brush contact the conditioning disc. The output speed of the motor for rotating the brush is less than or equal to 100 rpm.
The method of the present invention is performed as follows. First, a semiconductor wafer is set on the upper surface of the polishing pad. The wafer is pressed by the polishing head against the polishing pad to fix the wafer in place atop the pad. Slurry is introduced onto the polishing pad and the polishing pad is rotated. As a result, the slurry flows between the bottom surface of the wafer and the upper surface of the polishing pad, whereby the surface of the wafer is polished.
After the surface of the wafer is polished, the wafer is transferred by a robot arm or the like off of the polishing pad.
Now, the polishing of the wafer degrades the upper surface of the polishing pad. Therefore, after the wafer is transferred off of the polishing pad, the conditioning disc is run over the upper surface of the polishing pad to condition the surface of the polishing pad.
Once the upper surface of the polishing pad has been conditioned, the conditioning disc is moved off of the upper surface and into the liquid in the tank. The conditioning disc stands by as submerged in the liquid to prevent slurry or polishing by-products from hardening on the disc.
While the conditioning disc is at this stand-by position, polishing impurities are forced off of the abrasive surface and into the liquid by the conditioning disc cleaner. When the cleaner is a brush, the brush and/or the conditioning disc are rotated relative to each other while the bristles of the brush contact the abrasive surface of the conditioning disc. In this way, material that would otherwise act as an impurity during the next polishing apparatus is dislodged from the conditioning disc.